In a flashback, we get Joe Christmas's history. When he is five years old, Joe Christmas sneaks into an unspecified room, looking for a tube of toothpaste.
He spreads the toothpaste onto his finger, but before he can put it in his mouth he hears footsteps.
The dietitian enters the room with a man named Charley, and Christmas hides behind a curtain. Charley and the dietitian begin to have sex while Christmas hides.
Christmas keeps eating toothpaste until he gets sick and vomits.
When he vomits, the dietitian and the young doctor, Charley, discover him. The dietitian scolds Christmas and calls him a "little n***** bastard."
As days pass, the dietitian becomes convinced that Christmas is going to tell on her. One day she offers him a dollar if he keeps his mouth shut. Christmas doesn't understand that she means to give the dollar to him so he keeps silent and doesn't take it.
The dietitian plots to get revenge on the boy, convinced he's going to tell on her.
One afternoon, the dietitian approaches the janitor. She tells the janitor that she knows that he hates the boy too, because she's seen him spying on Christmas. She tells the janitor that he knows that the boy is biracial.
The janitor agrees but tells the dietitian that she cannot tell the matron yet; instead she must wait for God to find a way to tell the matron.
Later that night the janitor comes to the dietitian's room while she's undressing. He pushes his way into her room and asks her if she's told the matron yet. He says that the matron will send the boy to a black orphanage once she finds out the truth. He curses her, refers to her things as "womanfilth," and leaves the room.
The next morning the janitor and Christmas are gone – the janitor has kidnapped Christmas.
The dietitian tells the matron that Christmas is black, but she sure doesn't mention having sex with the doctor!
The narration now follows Christmas and the janitor. The janitor wakes up Christmas, dresses him, and takes him out of the orphanage. They get into a streetcar and then onto a train. They travel for three days before the police catch them and take Christmas back to the orphanage.
One day, two women clean Christmas up and ask him if he'd like to live with a nice family out in the country.
Christmas is introduced to a stern-looking man named McEachern, who says he will raise Joe to fear God. He says that Christmas is a heathen name.
Joe goes with McEachern to begin his new life in the country with his foster parents.